Ruth Plumly Thompson 
Kabumpo in Oz [EPUB ebook] 

Kabumpo in Oz is the sixteenth Oz book, and the second written by Ruth Plumly Thompson. During Prince Pompadore of Pumperdink’s eighteenth birthday celebration, his birthday cake explodes, revealing a magic scroll, a magic mirror, and a doorknob. The scroll warns the prince that if he doesn’t wed a "proper princess" within seven days, his entire kingdom will disappear. The prince, along with the kingdom’s wise elephant Kabumpo, set off on an adventure to the Emerald City so Pompa can marry Princess Ozma, the only "proper princess" the Elegant Elephant can think of as worthy of his prince.